Rotary atomizers are a type of liquid spray coating apparatus which includes an atomizer head rotatable at high speed (typically 10,000-40,000 revolutions per minute) to apply liquid coating material in atomized form onto the surface of a workpiece. The head is usually in the form of a disc or cup which includes an interior wall defining a cavity and terminating in an atomizing edge. Liquid coating material delivered to the interior of the cup migrates outwardly under centrifugal force along the wall until it is flung from the edge of the cup and thereby atomized. To improve the transfer efficiency of the coating process, an electrostatic charge is imparted to the coating material so that the atomized coating material is attracted to an electrically grounded workpiece. An example of an electrostatically charged type rotary atomizer is disclosed in commonly assigned U.S. Pat. No. 4,887,770 ('770) to Wacker et al., which is expressly incorporated herein in its entirety by reference. In the FIG. 12 embodiment of the '770 patent, the cup (20) is made from an insulative material and includes a semi-conductive ring (546) which is charged through posts (504) by three external electrode probes (462). This system suffers from a drawback in that the front end of the housing from which the cup protrudes has a large profile which causes the air currents, generated by the high speed rotation off the cup, to create a vacuum around the front end of the housing which in turn causes the paint to wrap back onto the housing. While this problem has been addressed by directing auxiliary air around the front end of the housing to break up the vacuum, there is a need for an atomizer which does not cause wrap back and does not create a safety hazard.
Prior to the '770 patent, one of the hazards associated with the use of the conductive atomizing cup was the possibility of operator shock or ignition of combustible coatings because of the high voltage at which the cups were maintained. For example, as disclosed in U.S. Pat. No. 4,369,924, charge is transferred through a turbine shaft from a power supply to the rotary atomizer cup. However, both the cup and the entire rotary atomizing housing are metal and are charged to a high voltage. With this type of construction, there is a significant safety hazard since the atomizer carries sufficient charge to shock an operator severely and therefore protective fences and interlocks have to be installed around the atomizer.
The '770 patent, listed before, teaches a low capacitance, rotary atomizer which while electrostatically charging the paint at the rotary atomizer cup does not store sufficient charge to present a shock hazard and therefore does not have to be protected by fences and safety interlocks. However, since the cup (20) is charged through external electrode probes (462), the system suffers from the drawback that the front end of the housing has a large profile with the attendant problems discussed before.
In another type of rotary atomizer, as disclosed in U.S. Pat. No. 3,826,425, a disk (11) comprised of both insulative and conductive parts, is charged by power supply (21) through cable (22), resistor (23), conductive foam (63), semi-conductive rod (62), semi-conductive ring (61), air gap (39), first semi-conductive plastic rod (24), semi-conductive plug (60), second semi-conductive plastic rod (24) and conductive ink (38). This system suffers from the drawback that the electrical path is made up of a large number of parts which increases the chance of system failure. Also, since the electrical path is spaced outwardly from the rotary shaft, the outer periphery of the front end of the housing has a large profile which causes the paint to wrap back onto the housing.